Within-host evolution and transmission of a human gut symbiont across ecological scales.

Adaptive mutations in Bacteroides thetaiotaomicron rapidly spread between co-housed mice, overcoming colonization resistance and enabling engraftment of specific lineages.

• Why it matters: Understanding how gut bacteria evolve and disperse across hosts is crucial for grasping their long-term success and stability within microbiomes.
• What they did: The study tracked approximately 70,000 genomically barcoded Bt lineages among co-housed mice, analyzing transmission rates, adaptive mutations, and community effects through whole-genome sequencing and in vitro evolution.
• The result: Shared selection pressures led to predictable lineage engraftment despite variable transmission rates, and community diversity shifted the adaptive landscape without slowing evolution, highlighting complex carbohydrate metabolism adaptations.